Switching power supply apparatus and semiconductor device

ABSTRACT

A switching power supply apparatus includes a PFM control circuit that outputs a clock signal Set such that a switching frequency of a switching element varies in accordance with a load state. The clock signal Set determines a turn-on timing of the switching element. A reference value of a current flowing through the switching element determines a turn-off timing of the switching element. A modulation signal is applied to the turn-off timing of the switching element to modulate one of a peak value of a drain current flowing through the switching element and an on-time of the switching element. Input control is performed separately on the clock signal Set and the modulation signal. Accordingly, even when the clock signal Set and the modulation signal contribute to each other to offset each other, modulation effects are not cancelled.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 14/275,348,filed on May 12, 2014, which is a continuation application of PCTApplication No. PCT/JP2012/004676 filed Jul. 24, 2012, designating theUnited States of America, which in turn claims the benefit of JapanesePatent Application No. 2011-258908, filed on Nov. 28, 2011, thedisclosures of which, including the specification, drawings and claims,is incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a switching power supply apparatusthat performs switching on an input voltage through a switching elementto control an output voltage, and to a semiconductor device thatconstitutes the switching power supply apparatus.

BACKGROUND ART

Conventionally, as a power supply apparatus for general appliances forhousehold use such as home electric appliances, a switching power supplyapparatus has been widely used that includes a semiconductor devicecontrolling an output voltage with use of a switching operation of asemiconductor (a switching element such as a transistor), for thepurpose of improvement of power efficiency by reduction in powerconsumption.

Especially in recent years, there has been a demand for reduction inpower consumption in electric appliances for prevention of globalwarming. Attention is particularly focused on power consumption in astandby mode of appliances having a standby function. There has been agreat demand for a switching power supply apparatus having a lower powerconsumption in the standby mode.

Generally, in a light load state such as the standby mode or the like, adominant energy loss in a switching power supply apparatus is aswitching loss due to a switching operation. As one of known arts forimproving the power efficiency in the light load state, power supplyoperation is performed under Pulse Frequency Modulation (PFM) control inwhich a switching frequency is decreased in accordance with a loadcurrent.

FIG. 17 shows a configuration example of a conventional switching powersupply apparatus that includes a semiconductor device having a PFMcontrol circuit.

As a load current output in a rated load state decreases, an outputvoltage increases. Regarding information indicating this increase of theoutput voltage, a feedback signal is input to an FB terminal via anoutput voltage detection circuit 5, and Pulse Width Modulation (PWM)control is performed such that a current flowing through a switchingelement 2 decreases in accordance with a value of a signal output from afeedback signal control circuit 11. A state under PWM controlcorresponds to a range A in FIG. 3, and a switching operation isperformed at 100 kHz for example.

When the load further decreases, the switching power supply apparatusswitches from PWM control to PFM control, and operates so as to vary aswitching frequency of the switching element 2 in accordance with theload state. A state under PFM control corresponds to a range B in FIG.3. PFM control is performed such that as the load decreases, a feedbackcurrent IFB increases, a voltage EAO decreases, and the switchingfrequency decreases. In this way, the switching power supply apparatusin a light load state reduces the number of switching operations byperforming PFM control to reduce the sum of switching losses as much aspossible, thereby to improve a power efficiency.

According to such a conventional switching power supply apparatus,however, when a load is constant, a switching frequency of a switchingelement operating a switching operation is fixed. This causes a problemthat spectral components of a high-frequency current flowing through theswitching element concentrate in the switching frequency and itsharmonic components, and as a result noise (electrical noise) easilyoccurs. Such noise is called a high-frequency noise or a terminal noise.Components such as filter circuits against the noise are necessary, andthis hinders size-reduction and cost-reduction of the switching powersupply apparatus.

Here, the terminal noise represents a leakage voltage that is induceddue to leakage of a switching frequency by a switching operation andharmonic components thereof from a commercial AC power supply to theoutside. A magnitude of the terminal noise is expressed by indicatorssuch as a peak value that is the maximum amplitude value of the noise, aquasi-peak (Qp) value that is close to the peak value and variesdepending on an amplitude and a frequency of the noise, and an averagevalue. When the switching frequency is constant, the peak value, the Qpvalue, and the average value are equal to each other with no variation.The standard value of the average value is set lower than the standardvalue of the Qp value. However, in the case where the Qp value and theaverage value are equal to each other as described above, the Qp valueneeds to be decreased to the average value.

Also, there is a case where when a load of a switching power supplyapparatus operating under PFM control decreases, the switching powersupply apparatus operates at an frequency in an audible frequency regionof 20 kHz or less (hereinafter, referred to as audible region). Inparticular, when there is a small variation in load in a standby mode orthe like, if a switching frequency is fixed at a particular frequency inthe audible region, sound is sometimes generated from a transformer, aceramic capacitor, and so on which are generally used in the switchingpower supply apparatus.

In response to this, a measure may be taken in which the minimumswitching frequency under PFM control is set to greater than 20 kHzbeyond the audible region. It is true that this measure preventsgeneration of sound in the transformer, the ceramic capacitor, and soon. Even in the light load state, however, the number of performance ofswitching increases due to the minimum switching frequency which is sethigh. This results in increase in switching loss to hinder improvementof power efficiency.

In addition to the above measure, there are known measures such as ameasure of performing PFM control by skipping the audible region, and ameasure of impregnating the transformer, the ceramic capacitor, and soon with a resin or the like. However, these measures cause increase inarea of circuits, increase in cost, and so on. As a result, there arisesa demand for improvement in the trade-off relationship betweenperformance and cost.

In response to the demand for suppression of noise occurrence asdescribed above, there have conventionally been proposed switching powersupply apparatuses such as disclosed in Patent Literatures 1 and 2. Theswitching power supply apparatuses such as disclosed in PatentLiteratures 1 and 2 can reduce an average value of a terminal noise bydiffusing a switching frequency of a switching element within apredetermined frequency range.

CITATION LIST Patent Literature [Patent Literature 1] Japanese PatentNo. 4461842 [Patent Literature 2] Japanese Patent ApplicationPublication No. 2008-312359 [Patent Literature 3] Japanese PatentApplication Publication No. 2009-142085 SUMMARY OF INVENTION TechnicalProblem

According to the switching power supply apparatus disclosed in PatentLiterature 1, when a load is fixed, a modulation signal is applied to aswitching frequency to diffuse frequency spectrum components, thereby toreduce the average value of the terminal noise. However, an A/Dconverter necessary for this decrease needs to be provided, and thisresults in increase in area of circuits and increase in cost.Furthermore, there is another problem that an effect is not exhibitedunless the load is fixed to a magnitude of a certain degree.

Also, according to the switching power supply apparatus disclosed inPatent Literature 2, a cyclic modulation signal is applied to aswitching frequency by a PFM frequency modulation circuit to diffusefrequency spectrum components, thereby to reduce the average value ofthe terminal noise. The increase in area of circuits and the increase incost are suppressed compared with the switching power supply apparatusdescribed in Patent Literature 1. However, a feedback signal and amodulation signal which vary in accordance with a load state are inputas elements for determining a turn-on timing of a switching element.When the load cyclically varies for example, the feedback signal and themodulation signal contribute to each other to offset each other, andmodulation effects are canceled. As a result, the modulation effects arenot exhibited or the switching power supply apparatus becomes unstableas a power supply apparatus due to excessive modulation.

Moreover, Patent Literatures 1 and 2 do not mention that a measureagainst sound generation of components such as a transformer and aceramic capacitor should be taken as a problem to be solved, despiteconcentration of the switching frequency in a specific frequency band inan audible region under PFM control.

The present disclosure was made in view of the above, and aims toprovide a switching source supply apparatus and a semiconductor deviceincluded therein that controls modulation of a switching frequency, inthe switching source supply apparatus operating under PFM control inwhich a switching frequency varies in accordance with a load state, inorder to stably and effectively an average value of a terminal noise andreduce sound generation of a transformer, a ceramic capacitor, and so ondue to operation in an audible frequency region.

Solution to Problem

In order to solve the above problem, a switching power supply apparatuscomprises: a transformer that includes a primary coil and a secondarycoil; a switching element that is connected in series with the primarycoil; a control circuit configured to control a switching operation ofthe switching element to perform switching control on a first DC voltagethat is input to the switching element via the primary coil; an outputvoltage generation unit configured to convert an AC voltage induced inthe secondary coil due to the switching operation to a second DCvoltage, and supply a power to a load; and an output voltage detectioncircuit configured to detect variation of the second DC voltage, andoutput a feedback signal generated in accordance with the variation tothe control circuit, the feedback signal being for the switching controlwherein the control circuit includes: a feedback signal control circuitconfigured to vary a switching frequency of the switching element inaccordance with the feedback signal output from the output voltagedetection circuit, such that the variation of the second DC voltage iscancelled; a PFM control circuit configured to generate a turn-on signalfor turning on the switching element at the switching frequencydetermined by the feedback signal control circuit; a current detectioncircuit configured to detect a value of a current flowing through theswitching element; a current control circuit configured to, when thevalue of the current detected by the current detection circuit reaches areference value, generate a turn-off signal for turning off theswitching element; and a current peak modulation unit configured tocyclically modulate a peak value of the current flowing through theswitching element within a current range of a first value to a secondvalue.

Here, the output voltage detection circuit outputs the feedback signalto control a turn-on timing of the switching element, the current peakmodulation unit outputs a modulation signal to control a turn-off timingof the switching element, and the control circuit separately performsinput control on the feedback signal and the modulation signal.

Advantageous Effects of Invention

According to the present disclosure as described above, even when thereis a small variation in load in a standby mode or the like and thefrequency is within an audible region, the switching frequency at thistime is modulated, and as a result energy of the switching frequency anda harmonic frequency thereof is diffused, and a peak value decreases. Asa result, a problem specific to the PFM control, in which sound isgenerated from a transformer, a capacitor, and so on during operationsin the audible region, is easily reduced with a comparative simpleconfiguration. This allows a switching power supply apparatus having alimited operating frequency range for fear of sound generation and so onto perform frequency operations in a wide operating frequency range.Also, it is possible to omit special measures against sound generationsuch as a measure of adhering the transformer and a measure ofimpregnating the transformer, and a special control circuit for reducingsound generation.

Also, input control is separately performed on an output signal fordetermining the switching frequency and a modulation signal formodulating the switching frequency. With this configuration, even whenthe output signal and the modulation signal contribute to each other tooffset variation thereof, modulation effects are not cancelled.Accordingly, compared with conventional arts, an average value of aterminal noise is stably and effectively reduced, thereby to lead toreduction of filter components against noise.

Also, under a conventional PFM control, when an input is low, when aload is heavy, when an output is high, when a capacitance of an inputelectrolytic capacitor is high, or the like, an input ripple voltageincreases. An input voltage varies in accordance with the increase ofthe input ripple voltage. As a result, modulation effects are exhibited.However, according to a DC input switching power supply apparatus, whenthe capacitance of the input electrolytic capacitor is low, when theinput is high where the input ripple voltage is low, when the load islight where the switching frequency tends to concentrate in an audibleregion, when the output is low, or when substantially no input ripplevoltage is induced, there is no diffusion of the switching frequency dueto the ripple voltage. Accordingly, the switching frequency of theswitching element is fixed as long as an output load is constant. As aresult, an average value of a terminal noise equals to the Qp value, andthis needs measures against noise. The present disclosure is highlyeffective in such a switching power supply apparatus.

The present disclosure allows modification of the switching frequency inany load state, and therefore exhibits a high effectiveness.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a switching power supply apparatusrelating to Embodiment 1.

FIG. 2 is a circuit diagram of a feedback signal control circuitrelating to Embodiment 1.

FIGS. 3A and 3B show, in a semiconductor device relating to Embodiment1, an example of relationship between a feedback current and a switchingfrequency and an example of relationship between a feedback current anda peak value of a drain current which can flow through a switchingelement.

FIG. 4 is a circuit diagram of a PFM control circuit relating toEmbodiment 1.

FIG. 5 is a circuit diagram of a modulation signal generation circuitrelating to Embodiment 1.

FIG. 6 is a circuit diagram of a low-frequency oscillator relating toEmbodiment 1.

FIG. 7 is a circuit diagram of a drain current limiting circuit relatingto Embodiment 1.

FIG. 8 is a circuit diagram of a turn-off control circuit relating toModification 1 of Embodiment 1.

FIG. 9 is a circuit diagram of a turn-off control circuit relating toModification 2 of Embodiment 1.

FIG. 10 is a circuit diagram of a turn-off control circuit relating toModification 3 of Embodiment 1.

FIG. 11 is a timing chart showing modulation of a switching element inthe switching power supply apparatus relating to Embodiment 1.

FIG. 12 is a timing chart showing modulation of a switching element in aswitching power supply apparatus relating to a conventional example.

FIG. 13 is a circuit diagram of a modulation signal generation circuitrelating to Modification 4 of Embodiment 1.

FIG. 14 is a circuit diagram of a switching power supply apparatusrelating to Embodiment 2.

FIG. 15 is a circuit diagram of a switching power supply apparatusrelating to Embodiment 3.

FIG. 16 shows an example of relationship between frequency spectrumcomponents and sound pressure of a transformer in PFM control.

FIG. 17 is a circuit diagram of a switching power supply apparatusrelating to a conventional example.

DESCRIPTION OF EMBODIMENTS

The following specifically describes a switching power supply apparatusand a semiconductor device as embodiments of present disclosure, withreference to the drawings.

Embodiment 1

FIG. 1 is a circuit diagram showing a configuration example of aswitching power supply apparatus that includes a semiconductor devicefor controlling a switching power relating to Embodiment 1.

As shown in FIG. 1, a transformer 1 includes a primary coil 1 a, asecondary coil 1 b, and an auxiliary coil 1 c. The primary coil 1 a andthe secondary coil 1 b are opposite in polarity to each other. Thisswitching power supply apparatus is of a flyback type.

A switching element 2 constituting part of a control circuit 3 isconnected with the primary coil 1 a. A switching operation of theswitching element 2 is controlled through variation of a voltage to beapplied to a control electrode (gate) of the switching element 2.

The switching element 2 and other part that constitute the controlcircuit 3 are integrated on the same semiconductor substrate, andconstitute a single semiconductor device. The switching element 2 is apower MOSFET or the like.

Note that the switching element 2 and the other part, which constitutethe control circuit 3, do not necessarily need to be arranged on thesame semiconductor substrate. For example, the following configurationmay be employed. The switching element 2 and the other part, whichconstitute the control circuit 3, are arranged on separate semiconductorsubstrates. On one of the semiconductor substrates on which the part,which constitute the control circuit 3, is arranged, an OUT terminal isprovided for outputting a signal output from the part. Furthermore, theOUT terminal is connected with the gate of the switching element 2 whichis arranged on the other semiconductor substrate.

The control circuit 3 includes, as external input and output terminals,four terminals of a DRAIN terminal, an FB terminal, a GND terminal, anda VCC terminal. The VCC terminal is a power source voltage terminal forpower supply to the control circuit 3.

The following briefly describes the control circuit 3. The controlcircuit 3 performs PFM control to control the switch operation of theswitching element 2, thereby to maintain a constant output voltage ofthe switching power supply apparatus. The control circuit 3 controls theswitching element 2 to perform the switch operation, by inputting aclock signal Set to S (set) of an RS flip-flop circuit 17. Also, thecontrol circuit 3 detects a current flowing through the switchingelement 2. When the detected current reaches a predetermined value, thecontrol circuit 3 inputs a turn-off signal to R (reset) to turn off theswitching element 2. As a result, the switch frequency of the switchingelement 2 is cyclically modulated.

The DRAIN terminal is a terminal that is connected with a connectionpoint between the primary coil 1 a of the transformer 1 and theswitching element 2, namely, a terminal connected with a drain of theswitching element 2.

The GND terminal is a terminal that connects each of a source of theswitching element 2 and GND of the control circuit 3 with a groundlevel. The GND terminal is connected with a terminal having a lowerpotential included in two terminals to which a DC voltage Vin isapplied.

Note that, in the present disclosure, the switching element 2 may be apower switching element other than the power MOSFET. In the case wherean insulated gate bipolar transistor (IGBT) is used for example,COLLECTOR and EMITTER of the IGBT correspond to DRAIN and SOURCE of thepower MOSFET.

The VCC terminal is a terminal that connects a rectifying and smoothingcircuit 4 constituted from a rectifying diode 4 a and a smoothingcapacitor 4 b with a regulator 8 included in the control circuit 3. TheVCC terminal rectifies and smoothes an AC voltage induced in theauxiliary coil 1 c due to the switching operation of the switchingelement 2, and inputs the AC voltage after rectifying and smoothing tothe control circuit 3 as an auxiliary power source voltage VCC.

The FB terminal is a terminal that inputs a feedback signal such as acurrent induced by a phototransistor, which is output from the outputvoltage detection circuit 5, to the feedback signal control circuit 11included in the control circuit 3.

Note that, instead of inputting the feedback signal to the FB terminal,the auxiliary power source voltage VCC, which results from rectifyingand smoothing the AC voltage induced in the auxiliary coil 1 c, may beinput to the feedback signal control circuit 11 included in the controlcircuit 3 via the output voltage detection circuit 5.

The regulator 8 is connected between the DRAIN terminal and the VCCterminal of the switching element 2, a start and stop circuit 10, and aninternal circuit voltage source 9 included in the control circuit 3.When an input DC voltage Vin is applied to the DRAIN terminal of theswitching element 2 via the transformer 1, the regulator 8 supplies acurrent from the DRAIN terminal to the capacitor 4 b of the rectifyingand smoothing circuit 4 via the VCC terminal to charge the smoothingcapacitor 4 b. This results in increase of the auxiliary power sourcevoltage VCC, which is a voltage output from the rectifying and smoothingcircuit 4.

When a voltage of the VCC terminal increases to a starting voltage ofthe control circuit 3, the regulator 8 suspends current supply from theDRAIN terminal to the VCC terminal. Then, a current is supplied from thesmoothing capacitor 4 b of the rectifying and smoothing circuit 4 to theinternal of the control circuit 3 via the VCC terminal. Here, theauxiliary power source voltage VCC is equivalent to a voltage resultingfrom rectifying and smoothing the voltage of the auxiliary coil 1 c,that is, a charging voltage of the smoothing capacitor 4 b. Also, whenthe voltage of the VCC terminal decreases to a stop voltage of thecontrol circuit 3, a current is supplied from the DRAIN terminal to theVCC terminal like before startup, and the voltage of the VCC terminalincreases again. The internal circuit voltage source 9 is controlled bythe regulator 8 so as to have a constant value.

The start and stop circuit 10 monitors the voltage of the VCC terminal,and controls the switching element 2 to start and stop in accordancewith a value of the voltage of the VCC terminal. When the voltage of theVCC terminal increases to the starting voltage, the start and stopcircuit 10 outputs an H level voltage to one of input terminals of aNAND circuits 18. When the voltage of the VCC terminal decreases to thestop voltage, the start and stop circuit 10 outputs an L level voltageto the other input terminal of the NAND circuit 18. Here, the H level isa predetermined voltage level that is higher than 0 V such as a powersource voltage VDD, and the L level is a ground (GND) level.

The feedback signal control circuit 11 outputs a voltage signal EAO forcontrolling the switching frequency of the switching element 2 so as tostabilize an output DC voltage Vout, in accordance with a feedbacksignal which is input to the FB terminal of the control circuit 3 fromthe output voltage detection circuit 5. The voltage output from thefeedback signal control circuit 11 performs control, such that when theload is light and the output DC voltage Vout increases, the switchingfrequency of the switching element 2 decreases, and when the load isheavy and the output DC voltage Vout decreases, the switching frequencyof the switching element 2 increases.

FIG. 2 shows the configuration of the feedback signal control circuit11.

In FIG. 2, reference numerals 71 and 72 each represent a constantcurrent source, reference numerals 73 and 74 each represent a P-MOSFET,and reference numerals 75, 76, 77, and 79 each represent an N-MOSFET.Also, reference numerals 78 and 82 each represent a constant voltagesource, reference numeral 80 represents a resistor, reference numeral 81represents an NPN bipolar transistor. The N-MOSFET 77, the resistor 80,the NPN bipolar transistor 81, and the constant voltage source 82constitute an I-V converter. The P-MOSFETs 73 and 74 constitute a mirrorcircuit, and the N-MOSFET 76 and 77 constitute a mirror circuit.

The constant current sources 71 and 72 each set current limitation whenthe FB terminal short-circuits with the GND. The voltage EAO which isconverted by the I-V converter is determined in accordance with currentflowing through the resistor 80, and varies as shown in Expression 1below.

[Math 1]

VEAO=VR−Vbe−R×I  Expression 1

Here, VEAO denotes an output voltage of the I-V converter, VR denotes aconstant voltage of the constant voltage source 82, Vbe denotes a B-Evoltage of the NPN bipolar transistor 81, R denotes a resistance of theresistor 80, and I denotes a current flowing through the resistance 80.

As clear from Expression 1, as the current I flowing through theresistance 80 increases, the output voltage EAO decreases. Also, thevoltage EAO which is output from the feedback signal control circuit 11is used for controlling the switching frequency for PFM control.

In other words, as the current flowing from the FB terminal increases,the output voltage EAO decreases and as a result the switching frequencyof the switching element 2 decreases. Also, as the current flowing fromthe FB terminal decreases, the output voltage EAO increases and as aresult the switching frequency of the switching element 2 increases.

FIG. 3A shows, an example of relationship between a feedback current IFBand the switching frequency of the switching element 2, and FIG. 3Bshows, an example of relationship between the feedback current IFB and apeak value of a drain current flowing through the switching element 2.

As shown in FIGS. 3A and 3B, the switching power supply apparatus, whichperforms PWM control and PFM control in combination, detects a loadstate by the feedback signal control circuit 11, and switches betweenPWM control and PFM control based on a predetermined value.

In a switching power supply apparatus such as shown in FIG. 17 forexample, a switching power supply apparatus operates under PFM controlwhen the load is lower than a predetermined value (light load), andoperates under PWM control (or quasi-resonant control) when the load isequal to or higher than the predetermined value (heavy load). Whencontrol switches completely from PFM control to PWM control as a resultof increase of the load to a certain degree, a frequency of a clocksignal Set output from a PWM control circuit is controlled to be a fixedvalue such as 100 kHz. As a result, a current flowing through aswitching element 2 is controlled to increase as the load increases inaccordance with the feedback signal output from an output voltagedetection circuit 5, that is, a feedback current IFB flowing from an FBterminal.

In the case where a frequency modulation unit for modulating the peakvalue of the drain current, which is included in the switching powersupply apparatus relating to the present embodiment, the switchingfrequency is corrected by a feedback response for maintaining a constantoutput due to variation of a peak value of a drain current. There is acase where when the feedback response (feedback signal) greatly delaysrelative to the switching frequency, an output of a load instantaneouslyvaries. For example, there is a case where when the load state under PFMcontrol is immediately before switching to the load state under PWMcontrol, the load state is judged to be under PWM control due to thevariation of the output of the load despite that the load state isactually to be under PFM control.

Also, under PWM control, even when the peak value of the current ismodulated, the switching frequency is fixed and is not modulated.Generally, as a modulation unit under PWM control, application of amodulation signal to the switching frequency is performed. Also, in thecase where quasi-resonance control and PFM control are used incombination, the switching frequency is modulated by modulating the peakvalue of the current even when the quasi-resonance control is used, andaccordingly it is unnecessary to switch the modulation unit. In the casewhere the PWM control and the PFM control are used in combination,however, the switching frequency is not modulated even if modulation ofthe peak value of the current is adopted as the modulation unit underPWM control, and accordingly it is necessary to switch the modulationunit under PWM control.

Also, there is a case where the switching power supply apparatusrepeatedly switches between PFM control and PWM control at a high speeddepending on the load state. In this case, switching of the modulationunit needs to be repeatedly performed at a high speed. This might resultin no substantial modulation effect and unstable operations of theswitching power supply apparatus.

In response to this problem, hysteresis control may be performed onswitching of an operation mode when switching is performed between PWMcontrol and PFM control, for example. Alternatively, a peak value of adrain current may be clamped.

A PFM control circuit 12 includes an oscillator that outputs a clocksignal Set for turning on the switching element 2. The PFM controlcircuit 12 varies a frequency of the clock signal Set so as to maintainan output voltage Vo of the switching power supply apparatus at aconstant value. The PFM control circuit 12 varies the frequency of theclock signal Set in accordance with a value of a conversion voltage EAOwhich is input from a feedback signal control circuit 11. Specifically,when the conversion voltage EAO increases, the PFM control circuit 12increases the frequency of the clock signal Set. Conversely, when theconversion voltage EAO decreases, the PFM control circuit 12 decreasesthe frequency of the clock signal Set. The clock signal Set controls aturn-on timing of the switching element 2 thereby to vary the switchingfrequency of the switching element 2. This maintains the output voltageVo of the switching power supply apparatus at a constant value.

FIG. 4 shows the PFM control circuit 12 relating to Embodiment 1. ThePFM control circuit 12 includes a V-I converter 118, a constant currentsource 101, and an oscillator 100. As shown in FIG. 4, elements areconnected with each other. The V-I converter 118 converts a conversionvoltage EAO corresponding to a feedback signal to a current signal IEAO.The current signal IEAO is superimposed on a constant current of theconstant current source 101. A current signal resulting fromsuperimposing the current signal IVEAO on the constant current of theconstant current source 101 is input to the oscillator 100. A clocksignal Set is output from the oscillator 100 via a pulse generator. As aresult, since the frequency of the clock signal Set varies in accordancewith variation of the load 7, the switching frequency of the switchingelement 2 also varies such that the output voltage Vo of the switchingpower supply apparatus is maintained constant.

When the clock signal Set which is input to the set (S) rises, the RSflip-flop circuit 17 switches to a set state. Also, when a reset signalis input to the reset (R) via an AND circuit 15, the RS flip-flopcircuit 17 switches to a reset state. The RS flip-flop circuit 17generates an output signal (first logic signal) having a level thatswitches between the H level and the L level, in accordance with whetherthe RS flip-flop circuit 17 is in the set state or the reset state. Inother words, when the RS flip-flop circuit 17 is in the set state, thesignal level of the output signal is high, and when the flip-flopcircuit 17 is in the reset state, the signal level of the output signalis low.

A gate driver 19 generates a drive output signal for driving a controlterminal (gate terminal) of the switching element 2 based on an outputsignal from the NAND circuit 18. Specifically, when a voltage level ofthe drive output signal of the gate driver 19 reaches the H level, theswitching element 2 is turned on. When the voltage level of the driveoutput signal of the gate driver 19 reaches the L level, the switchingelement 2 is turned off.

The NAND circuit 18 generates an operational signal indicating acalculation result of the signal output from the RS flip-flop circuit 17and an output signal from the regulator 8 which is described later.

In a starting state, an H level signal is output from the start and stopcircuit 10, and accordingly an H level signal is input to one of theinput terminals of the NAND circuit 18. Also, the clock signal Set iscyclically output from the PFM control circuit 12 in accordance with theload state, and accordingly an H level pulse signal is input to the set(S) of the RS flip-flop circuit 17. As a result, the output (Q) is atthe H level, and an H level signal is also input to the other inputterminal of the NAND circuit 18. Since an L level signal is output fromthe NAND circuit 18, an H level signal is output from the gate driver19. As a result, the switching element 2 turns on.

A drain current detection circuit 20 functioning as a detection circuitfor detecting a current flowing through a switching element is connectedwith the DRAIN terminal. The drain current detection circuit 20 detectsan ON voltage determined based on a product of a drain current flowingthrough the switching element 2 and an ON resistance of the switchingelement 2 thereby to relatively detect a drain current flowing throughthe switching element 2. Then, the drain current detection circuit 20generates a voltage signal Vis that is proportional to a value of thedrain current, and outputs the generated voltage signal Vis to a plus(+) side of the comparator 14. When the voltage signal Vis equals apredetermined reference value, the comparator 14 outputs an H levelsignal to one of input terminals of the AND circuit 15.

Also, in the case where the switching element 2 and the control circuit3 are formed in separate substrates, a sense resistor may be provided ona source (ground side) of the switching element 2 (specifically, thepower MOSFET or the like) to detect a potential difference of the senseresistor and output a voltage signal corresponding to the potentialdifference to the plus side of the comparator 14.

After the gate driver 19 outputs a turn-on signal of the switchingelement 2, an on-time blanking pulse generation circuit 16 sets acertain blanking time so as not to erroneously detect a capacitive spikecurrent and so on induced due to the capacitance of the switchingelement 2.

After elapse of the blanking time, the on-time blanking pulse generationcircuit 16 outputs an H level signal to one of input terminals of theAND circuit 15.

A drain current limiting circuit 21 compares a predetermined referencevoltage with a voltage Vis which is output from the drain currentdetection circuit 20. When the voltage Vis reaches the reference voltageand the switching element 2 turns on, an H level signal is input to bothof the input terminals of the AND circuit 15 after elapse of theblanking time set by the on-time blanking pulse generation circuit 16.Accordingly, an H level signal is output from the AND circuit 15, and isinput to the reset (R) of the RS flip-flop circuit 17. The drain currentlimiting circuit 21 includes a turn-off control circuit 150.

As a result, the output (Q) switches to the L level, and an L levelsignal is input to one of the input terminals of the NAND circuit 18.Accordingly, an L level signal is output from the gate driver 19, andthe switching element 2 turns off.

The turn-off control circuit 150 receives input of a signal Vis, areference value signal, and a signal Jitter, and outputs a turn-offcontrol signal OFF. Here, the signal Jitter is generated by a modulationsignal generation circuit 13 shown in FIG. 5. The modulation signalgeneration circuit 13 converts a triangular wave voltage which isgenerated by a low-frequency oscillator 50 to a current signal Jitterwith use of a V-I converter, and outputs the signal Jitter as a cycliccurrent modulation signal.

FIG. 7 shows the configuration of the turn-off control circuit 150.

Reference numeral 90 represents an operational amplifier, referencenumeral 91 represents a resistor, and reference numeral 14 represents acomparator. The reference voltage, which is input from the feedbacksignal control circuit 11, is impedance-converted by the operationalamplifier 90. The signal Jitter, which is input from the modulationsignal generation circuit 13, is input as a voltage of a minus (−) sideof the comparator 14, as the sum of a voltage induced due to a currentflowing through the resistor 91 and the reference voltage. The signalVis, which is input from the drain current detection circuit 20, isinput as a voltage of the plus side of the comparator 14. When thesignal Vis exceeds the reference voltage, the turn-off control signalOFF is input to the reset (R) of the RS flip-flop circuit 17, and as aresult the switching element 2 turns off.

Through the above signal processing, the switching element 2 performsthe switching operation. Also, an output voltage generation unit 6 thatis constituted from a rectifying diode 6 a and a smoothing capacitor 6 bis connected with the secondary coil 1 b. The output voltage generationunit 6 rectifies and smoothes an AC voltage induced in the secondarycoil 1 b due to the switching operation of the switching element 2,thereby to generate an output DC voltage Vout for supply to the load 7.

Also, the output voltage detection circuit 5 is made from an LED, aZener diode, or the like. The output voltage detection circuit 5 detectsthe voltage level of the output DC voltage Vout, and outputs a feedbacksignal necessary for the control circuit 3 to control the switchingoperation of the switching element 2, so as to stabilize the output DCvoltage Vout at a predetermined value.

According to this switching power supply apparatus, an AC power of acommercial AC power supply is rectified by a rectifier such as a diodebridge, and then is smoothed by an input capacitor, thereby to beconverted to a DC voltage Vin. The DC voltage Vin is applied to theprimary coil 1 a of the transformer 1 for power conversion.

The following describes the operation of the switching power supplyapparatus with the configuration shown in FIG. 1 and the semiconductordevice for controlling the switching power supply apparatus.

When an AC power of a commercial AC power supply is input to therectifier such as a diode bridge, the AC power is rectified and smoothedrespectively by the rectifier and the input capacitor, thereby to beconverted to a DC voltage Vin. The DC voltage Vin is applied to theDRAIN terminal via the primary coil 1 a of the transformer 1, and acharging current for startup flows through the smoothing capacitor 4 b,which is connected with the VCC terminal, from the DRAIN terminal viathe regulator 8 included in the control circuit 3. When the voltage ofthe VCC terminal of the control circuit 3 reaches the starting voltageset by the start and stop circuit 10 due to the charging current, thecontrol circuit 3 starts controlling the switching operation of theswitching element 2.

Also, since the output voltage Vout at the secondary side is low atstartup, the feedback signal output from the output voltage detectioncircuit 5 is not input to the feedback signal control circuit 11.Accordingly, the I-V converter included in the feedback signal controlcircuit 11 outputs a high converted voltage EAO, and oscillation startsat a high switching frequency and at a high peak value of the draincurrent under PWM control. In order to avoid this, a soft start functionis provided for example such that the switching frequency and the peakvalue of the drain current gradually increase only at startup.

Once the switching element 2 turns on, a current flows through theswitching element 2, and a voltage Vis corresponding to a value of thecurrent flowing through the switching element 2 is input to the plusside of the comparator 14. After elapse of a blanking time set by theon-time blanking pulse generation circuit 16, when a signal Vis outputfrom the drain current detection circuit 20 reaches a reference voltagewhich is determined at the minus side of the comparator 14, an H levelsignal is input to both of the input terminals of the AND circuit 15.Accordingly, an H level signal is output from the AND circuit 15 to thereset (R) of the RS flip-flop circuit 17, and the switching element 2turns off.

When the switching element 2 turns off, energy stored in the primarycoil 1 a of the transformer 1 during the on-time of the switchingelement 2 is transmitted to the secondary coil 1 b.

As a result of repetition of the switching operation as described above,the output DC voltage Vout increases. When the output DC voltage Voutreaches a voltage value which is set by the output voltage detectioncircuit 5, the output voltage detection circuit 5 performs control toflow a current from the FB terminal of the control circuit 3 as afeedback signal. The voltage EAO, which is converted by the I-Vconverter included in the feedback signal control circuit 11, decreasesin accordance with a value of the current. The frequency of the clocksignal Set is controlled to decrease in accordance with the decrease ofthe voltage EAO, thereby to adjust the switching frequency of theswitching element 2.

In this way, the output DC voltage Vout is varied to an appropriatevalue. In other words, the switching element 2 turns on in accordancewith the clock signal Set that is a pulse output from the PFM controlcircuit 12, and turns off when the current flowing through the switchingelement 2 reaches the reference value determined beforehand by the draincurrent limiting circuit 21.

That is, in a light load state where a low current is supplied to theload 7, the switching frequency, which is the number of times thecurrent flows through the switching element 2, decreases, and in a highload state, the switching frequency, which is the number of times thecurrent flows through the switching element 2, increases. As describedabove, the control circuit 3 performs control to vary the switchingfrequency of the switching element 2 in accordance with a power suppliedto the load 7.

The reference voltage at the minus side of the comparator 14 cyclicallyvaries within a certain range in accordance with a current signal Jitterwhich is output from the modulation signal generation circuit 13 and iscyclically modulated. As a result, the turn-off timing of the switchingelement 2 is modulated. The modulation signal generation circuit 13 isdescribed in detail later, and accordingly description thereof isomitted here.

As described above, cyclic modulation of the reference voltage resultsin cyclic modulation of the peak value ILIMIT of the drain current. As aresult, cyclic modulation of the switching frequency results indispersion of the spectrum components of the switching frequency.

The following simply describes that the switching frequency of theswitching element 2 is modulated as a result of cyclic modulation of thepeak value of the drain current.

With respect to the peak value ILIMIT of the current flowing through theswitching element 2, a relational expression shown in Expression 2 issatisfied.

[Math 2]

P=Io×Vo=½×L×ILIMIT² ×f×η[W]   Expression 2

According to Expression 2, when a load current Io is constant, variationof the peak value ILIMIT of the current flowing through the switchingelement 2 causes variation of the switching frequency f of the switchingelement 2. For example, as the peak value ILIMIT of the currentincreases, the switching frequency f decreases. Also, as the peak valueILIMIT of the current decreases, the switching frequency f increases.Accordingly, when the peak value of the current flowing through theswitching element 2 cyclically and continuously varies within a currentrange of a first peak value to a second peak value, the switchingfrequency f cyclically and continuously varies within a frequency rangeof a first switching frequency to a second switching frequency inaccordance with the variation of the peak value of the current.

In other words, the turn-off control circuit 150 modulates the turn-offtiming of the switching element 2, thereby to modulate the peak value ofthe current flowing through the switching element 2. As a result, theswitching frequency does not concentrate in a certain frequency in allthe frequency operating range (all the load range) under PFM controleven if the input voltage and the load are constant. This leads todiffusion of switching noise.

The switching power supply apparatus relating to the present embodimentincludes the PFM control circuit 12 that outputs the clock signal Setsuch that the switching frequency of the switching element 2 varies inaccordance with the load state. The turn-on timing of the switchingelement 2 is determined by the clock signal Set. The turn-off timing ofthe switching element 2 is determined by the reference value of thecurrent flowing through the switching element 2. Input control isperformed separately on the clock signal Set and the modulation signal,such that the turn-on timing is determined by the modulation signal, andthe turn-off timing is determined by the clock signal Set throughapplication of the modulation signal to the turn-off timing of theswitching element 2. Accordingly, the feedback signal and the modulationsignal do not contribute to each other to offset variation thereof. Itis possible to stably and effectively modulate the peak value of thedrain current flowing through the current switching element 2 thereby tocyclically modulate the switching frequency.

FIG. 11 is a timing chart showing modulation of the switching elementunder PFM control relating to the present embodiment. FIG. 12 is atiming chart showing modulation of the switching element under aconventional PFM control.

According to the present disclosure as shown in FIG. 11, a feedbacksignal and a modulation signal do not cancel each other, and accordinglyfrequency dispersion occurs in any circumstances. As a result, a cycleof oscillation frequency varies in cycles of T1, T2, T3, . . . as shownin FIG. 11. Therefore, the switching frequency is modulated bymodulating a peak value of a current flowing through the switchingelement 2 between Id1 and Id3. This easily reduces the average value ofterminal noise due to leakage of a switching frequency of a switchingoperation and harmonic components thereof from the commercial powersupply to the outside.

Furthermore, it is possible to reduce sound generation of thetransformer 1 due to the switching operation in an audible frequencyregion under PFM control, by dispersing spectrum components of thefrequency in the same manner as described above.

According to a conventional PFM control as shown in FIG. 12, a feedbacksignal and a modulation signal cancel each other, and accordingly acycle of oscillation frequency is fixed to cycle T1 even if a turn-onsignal is modulated. As a result, there often occurs a case where aswitching frequency and harmonic component thereof are not appropriatelydispersed.

FIG. 16 shows a relationship between a fundamental wave switchingfrequency fl of a clock signal Set output from the PFM control circuit12 and a sound pressure of the transformer 1. The sound pressure of thetransformer 1 on the vertical axis expresses an absolute amount of alevel of sound of the transformer 1. The sound pressure of thetransformer 1 here expresses fundamental wave frequency components andharmonic component of the sound pressure of the transformer 1.

According to the present embodiment as shown in FIG. 16, the fundamentalwave switching frequency fl disperses and a peak value of the soundpressure of the transformer 1 decreases, compared with a conventionalswitching power supply apparatus. Accordingly, sound generation of thetransformer 1 is reduced compared with the conventional switching powersupply apparatus.

Sound generation of the transformer 1 is considered to occur due to thatspectrum components of the switching frequency coincide with a resonancefrequency of the transformer 1. With respect to the switching frequencyof the switching element 2, only by avoiding a fundamental waveswitching frequency from existing in an audible frequency region of 2kHz to 20 kHz which is generally considered to be avoided, it isimpossible to avoid a harmonic which is equal to an integral multiple ofa fundamental wave switching frequency from existing in the audiblefrequency region. Therefore, sound generation cannot be prevented.Harmonic components are generated which are equal to an integralmultiple, such as two times, three times, and n times, of thefundamental wave switching frequency. As the order of the integralmultiple increases, the spectral intensity decreases. When harmoniccomponents concentrate in a certain part of an audible region, soundgeneration occurs. For this reason, as shown in FIG. 16, even iffundamental wave switching frequency components are avoided fromexisting in an audible region, sound generation of the transformer 1occurs with a small sound pressure unless harmonic components areavoided from existing in the audible region.

According to the present embodiment compared with this, control is notlimited to diffusion of the fundamental wave switching frequency in theaudible region and avoidance of the fundamental wave switching frequencyfrom existing in the audible region. The frequency is diffused in a widefrequency band from the fundamental wave to the harmonic of theswitching frequency. Therefore, it is possible to effectively preventsound generation of the transformer compared with conventional arts forreducing sound generation.

Here, the modulation signal generation circuit 13 is described in detailwith reference to FIG. 5.

The modulation signal generation circuit 13 outputs a current signalJitter resulting from converting a triangular wave voltage which isoutput from the low-frequency oscillator 50 to a current by a V-Iconverter which is configured by an NPN bipolar transistor 51, aresistor 52, and P-MOSFETs 53 and 54.

The low-frequency oscillator 50 is described with reference to FIG. 6.

In FIG. 6, numeral references 59, 60, and 61 each represent a constantcurrent source, numeral reference 67 represents a capacitor, numeralreferences 62 and 63 each represent a P-MOSFET, numeral references 64and 65 each represent an N-MOSFET, numeral reference 66 represents aninverter circuit, numeral reference 68 represents a resistor, andnumeral reference resistance 69 represents a comparator. The N-MOSFETs64 and 65 constitute a mirror circuit. Note that a voltage Va at a minusside of the comparator 69 is determined by the resistor 68 and theconstant current sources 60 and 61. The voltage Va satisfies Va=I₁×R₀when the P-MOSFET 70 is off, and satisfies Va=(I₁+I₂)×R₀ when theP-MOSFET 70 is on, where R₀ denotes a resistance of the resistor 68, andI₁ and I₂ respectively denote current values of the constant currentsources 60 and 61.

The following describes the operation of the low-frequency oscillator 50having the configuration shown in FIG. 6.

When an L level signal is output from the comparator 69, the P-MOSFETs63 and 70 are on. Also, since an H level signal is input to the gate viathe inverter circuit 66, the P-MOSFET 62 is off. Here, the voltage Va ata minus side of the comparator 69 satisfies Va=(I₁+I₂)×R₀. Also, sincethe P-MOSFET 63 is on, a constant current I₀ of the constant currentsource 59 flows to the capacitor 67 via the P-MOSFET 63. This results inincrease of a voltage Vb at a point b that is a plus side of thecomparator 69. When the voltage Vb at the point b exceeds the voltageVa=(I₁+I₂)×R₀ at the point a, the output signal of the comparator 69switches to an H level signal, and as a result the P-MOSFETs 63 and 70switch off. Here, the voltage Va at the minus side of the comparator 69satisfies Va=(I₁+I₂)×R₀.

Also, since an L level signal is input to the gate via the invertercircuit 66, the P-MOSFET 62 is on. When the P-MOSFET 62 switches on, theconstant current I₀ of the constant current source 59 flows to theN-MOSFET 64 via the P-MOSFET 62. The N-MOSFETs 64 and 65 constitute amirror circuit. For this reason, if the mirror circuit has a minor ratioof 1 for example, a current flowing through the N-MOSFET 65 is also theconstant current I₀.

Therefore, the electrical charge stored in the capacitor 67 is extractedat the constant current I₀, and as a result the voltage Vb at the pointb decreases. When the voltage Vb at the point b decreases to the voltageVa=I₁×R₀ at the point a, an L level signal is again output from thecomparator 69. A time T_(M) that is a time as one cycle of thetriangular wave voltage is expressed as the following Expression 3,where C₀ denotes the capacitance value of the capacitor 67.

$\begin{matrix}\left\lbrack {{Math}\mspace{14mu} 3} \right\rbrack & \; \\{T_{M} = {2 \times \frac{C_{0} \times \left( {I_{2} \times R_{0}} \right)}{I_{0}}}} & {{Expression}\mspace{14mu} 3}\end{matrix}$

As a result of repetition of the operation as described above, thetriangular wave voltage, which is output from the low-frequencyoscillator 50, continuously varies within a voltage range (I₂×R₀) of afirst voltage value (I₁×R₀) to a second voltage value ((I₁+I₂)×R₀) at acycle T_(M).

In this way, the generated triangular wave voltage Vf (t), which isoutput from the low-frequency oscillator 50 is input to the V-Iconverter included in the modulation signal generation circuit 13, andthereby to be modulated to the current Jitter. In accordance with thismodulation, the peak value ILIMIT of the current flowing through theswitching element 2 is modulated. Therefore, even if the input voltageVin and the load current Io are constant, the switching frequency f doesnot concentrate in a certain frequency, and thereby switching noise isdiffused.

Note that the cycle T_(M) of the low-frequency oscillator 50 shoulddesirably be several hundred Hz to several kHz.

Modifications of Embodiment 1 Modification 1

A switching power supply apparatus relating to Modification 1 ofEmbodiment 1 is substantially the same as the switching power supplyapparatus relating to Embodiment 1, except for a method of modulatingthe peak value of the current flowing through the switching element.

FIG. 8 is a circuit diagram of a turn-off control circuit 150 relatingto Modification 1 of Embodiment 1. Reference numerals 151, 156, and 157each represent an inverter circuit, reference numeral 152 represents aconstant current source, reference numeral 153 represents a P-MOSFET,reference numerals 154, 158, and 159 each represent an N-MOSFET, andreference numeral 155 represents a smoothing capacitor. The N-MOSFETs158 and 159 constitute a mirror circuit.

In the turn-off control circuit 150 relating to Embodiment 1, the peakvalue of the current flowing through the switching element 2 ismodulated by modulating the reference voltage input as a voltage at theminus side of the comparator 14. In the turn-off control circuit 150relating to Modification 1 compared with this, the peak value of thecurrent flowing through the switching element 2 is modulated byproviding a delay time for an output signal OFF of a comparator 14, andmodulating the delay time.

The following describes the operation of a drain current limitingcircuit including the turn-off control circuit 150.

When a signal for limiting the peak value of the current flowing throughthe switching element 2 is input to a delay time generation circuit 160from the comparator 14, the P-MOSFET 153 switches on, and a chargingcurrent flows through the smoothing capacitor 155 from the constantcurrent source 152. When the inverter circuit 156 inverts, the delaytime generation circuit 160 outputs a turn-off control signal OFF. Inother words, the switching element 2 does not turn off immediately afterthe peak value of the current flowing through the switching element 2reaches a value corresponding to the feedback signal. The switchingelement 2 turns off when the delay time elapses after the peak value ofthe current reaches the corresponding value.

The current corresponding to the current signal Jitter, which is outputfrom the modulation signal generation circuit 13, is subtracted from thecurrent for charging the capacitor of the delay time generation circuit160, thereby to modulate the delay time. As a result, the peak value ofthe drain current flowing through the switching element 2 is modulated.

The switching power supply apparatus relating to Modification 1 ofEmbodiment 1 exhibits substantially the same effects as those of theswitching power supply apparatus relating to Embodiment 1.

Also, the delay time generation circuit 160 may be provided downstreamof the AND circuit 15, instead of downstream of the comparator 14, so asto provide a delay time for a signal which is input to the reset (R) ofthe RS flip-flop circuit 17.

Here, the modulation signal generation circuit 13 shown in FIG. 5 isdescribed.

Assume here that the mirror ratio of the P-MOSFETs 53 and 54constituting the mirror circuit is 1:1, for example. A current If (t)flowing in the on-state to the capacitor 155 is expressed as thefollowing Expression 4, where Vf (t) denotes a voltage output from thelow-frequency oscillator 50, R₀ denotes a resistance of the resistor 52,Vbe₀ denotes a B-E voltage of the NPN bipolar transistor 51, and I₀denotes a constant current of the constant current source 152.

$\begin{matrix}\left\lbrack {{Math}\mspace{14mu} 4} \right\rbrack & \; \\{{{If}(t)} = \frac{{{Vf}(t)} - {Vbe}_{0}}{R_{0}}} & {{Expression}\mspace{14mu} 4}\end{matrix}$

Since the output voltage Vf (t) cyclically and continuously varieswithin the voltage range of the first voltage value to the secondvoltage value, the current If (t) cyclically and continuously varieswithin the current range of the first current value to the secondcurrent value.

Charging of the capacitor 155 with the current If (t) increases thepotential of the capacitor 155. When the potential exceeds a thresholdvoltage value of the inverter circuit 156, an output signal of theinverter circuit 156 switches to an L level signal. An H level signal isoutput as an output signal OFF via the inverter circuit 157.

A delay time tf (t) is expressed as the following Expression 5, which isa time from when the P-MOSFET 153 switches on to when the output of theinverter circuit 156 switches from the H level to the L level, in otherwords, a delay time from when an H level signal is input to the currentdetection terminal to when the output terminal OFF switches to the Hlevel.

$\begin{matrix}\left\lbrack {{Math}\mspace{14mu} 5} \right\rbrack & \; \\{{{tf}(t)} = \frac{C \times {Vt}}{{If}(t)}} & {{Expression}\mspace{14mu} 5}\end{matrix}$

Here, Vt denotes the threshold voltage value of the inverter circuit156, and C denotes the capacitance value of the capacitor 155.

As described above, the triangular wave voltage Vf (t), which is outputfrom the low-frequency oscillator 50, varies within the voltage range ofthe first voltage value to the second voltage value, and accordingly thecurrent If (t) cyclically and continuously varies within the currentrange of the first current value to the second current value.

As a result, the delay time tf (t) of the signal cyclically andcontinuously varies within a time range of a first delay time to asecond delay time, as clear from Expression 5. In other words, an Hlevel signal, which is output from the comparator 14, is input to theAND circuit 15 via the turn-off control circuit 150 after elapse of thedelay time tf (t). This turns off the switching element 2.

As described above, the switching element 2 does not turns offimmediately after detection of the current flowing through the switchingelement 2, but actually turns off when the delay time tf (t), which isdetermined by the turn-off control circuit 150, elapses after detectionof the current. In other words, the amount of the current actuallyflowing through the switching element 2 is determined in accordance withthe delay time tf (t) after current detection, the input voltage Vin,and a primary inductance LP of the transformer 1.

As described above, since the delay time tf (t) cyclically andcontinuously varies within the time range of the first delay time to thesecond delay time, the current flowing through the switching element 2cyclically and continuously varies within the current range of the firstcurrent value to the second current value.

In this way, the triangular wave voltage Vf (t), which is output fromthe low-frequency oscillator 50, is modulated thereby to modulate thecurrent If (t), which is converted by the V-I converter included in themodulation signal generation circuit 13. In accordance with thismodulation, the delay time tf (t) after detection of the current flowingthrough the switching element 2 is modulated. As a result, the peakvalue of the drain current flowing through the switching element 2 ismodulated. Therefore, even if the input voltage Vin and the load currentIo are constant, switching noise is diffused with no concentration ofthe switching frequency f in a certain frequency by modulating the peakvalue of the drain current flowing through the switching element 2.

Modification 2

FIG. 9 is a circuit diagram of a turn-off control circuit 150 relatingto Modification 2 of Embodiment 1.

The turn-off control circuit 150 adds a current for charging a capacitorof a delay time generation circuit 160 and a current corresponding to acurrent signal Jitter which is output from a modulation signalgeneration circuit 13, thereby to modulate a delay time.

Modification 3

FIG. 10 is a circuit diagram of a turn-off control circuit 150 relatingto Modification 3 of Embodiment 1.

The turn-off control circuit 150 modulates an output signal Vis of adrain current detection circuit 20 which is input as a voltage of a plusside of a comparator 14, thereby to modulate a peak value of a currentflowing through a switching element 2. The operating principle ofmodulation is substantially the same as that of Embodiment 1, andaccordingly detailed description thereof is omitted here.

The switching power supply apparatus relating to Modification 3 ofEmbodiment 1 exhibits substantially the same effects as those of theswitching power supply apparatuses relating to Embodiment 1 andModifications 1 and 2 of Embodiment 1.

Modification 4

A switching power supply apparatus relating to Modification 4 ofEmbodiment 1 is substantially the same as the switching power supplyapparatus relating to Embodiment 1, except for a method of generating amodulation signal.

FIG. 13 shows an example of a specific circuit configuration of amodulation signal generation circuit 13 included in a switching powersupply apparatus relating to Modification 4 of Embodiment 1.

Reference numerals 201 and 203 each represent an inverter circuit,reference numerals 202 and 204 each represent a D flip-flop circuit,reference numerals 205 and 207 each represent a constant current source,and reference numerals 206 and 208 each represent a P-MOSFET.

In the modulation signal generation circuit 13 included in the switchingpower supply apparatus relating to Embodiment 1, the triangular waveoutput from the low-frequency oscillator 50 is output as a modulationsignal after conversion by the V-I converter. The cycle of themodulation signal is equal to the cycle of the low-frequency oscillator50. In the modulation signal generation circuit 13 a included in theswitching power supply apparatus relating to Modification 4 comparedwith this, the number of times the triangular wave generated by thelow-frequency oscillator 50 reaches the upper limit is counted by acount-up circuit which is configured by the D flip-flop circuit forexample, and a current corresponding to the counted number of times isoutput as a modulation signal. In other words, the cycle of themodulation signal output from the modulation signal generation circuit13 a is equal to an integral multiple of the cycle of the low-frequencyoscillator 50.

This makes it possible to easily lengthen the cycle of the modulationsignal. Accordingly, even when the switching frequency is low under PFMcontrol, a modulation signal can be easily generated which has a cyclesufficiently longer than the switching frequency at this time. Forexample, it is possible to decrease a capacitance value of the capacitor67 included in the low-frequency oscillator 50, thereby exhibiting aneffect of reducing the size of semiconductor chip.

Note that, as the capacitor 67 included in the low-frequency oscillator50, a capacitor may be used which is connected with an externalterminal, instead of a capacitor provided in the semiconductor device.With this configuration, it is possible not only to easily generate amodulation signal having a relatively long cycle, but also to externallyperform adjustment in accordance with the specifications of theswitching power supply apparatus.

According to Modification 4, the switching frequency of the switchingelement 2 is cyclically and discretely (digitally) modulated by thecount-up circuit, compared with Embodiment 1 and Modifications of 1 to 3according to which the switching frequency of the switching element 2 iscyclically and continuously modulated. In other words, according toModification 4, it is possible to cyclically modulate the switchingfrequency by performing discrete (digital) step-up or step-down of thepeak value of the current per switching.

As described above, the switching power supply apparatus relating toModification 4 of Embodiment 1 exhibits substantially the same effectsas those of the switching power supply apparatuses relating toEmbodiment 1 and Modifications 1 to 3 of Embodiment 1.

Embodiment 2

The following describes a switching power supply apparatus relating toEmbodiment 2 of the present disclosure.

FIG. 14 is a circuit diagram of the switching power supply apparatusrelating to present embodiment.

The switching power supply apparatus relating to the present embodimentdiffers in terms of the following point from the switching power supplyapparatus relating to Embodiment 1 shown in FIG. 1. In the switchingpower supply apparatus relating to Embodiment 1, the drain current isdetected by the drain current detection circuit 20. When the currentreaches the set reference value, the switching element 2 turns off. Inthe switching power supply apparatus relating to the present embodimentcompared with this, an on-time generation circuit 22 is provided foralways setting a fixed on-time irrespective of a value of a currentflowing through a switching element 2. When turn-on of the switchingelement 2 is detected, a turn-on signal is input to an on-timegeneration circuit 22, and an on-time that is a time at which thecurrent flows through the switching element 2 reaches the value set bythe on-time generation circuit 22, the on-time generation circuit 22generates a signal for turning off the switching element 2.

Through the above control, PFM control is realized with a fixed on-time.Also, like in Embodiment 1, by varying the switching frequency inaccordance with the load state (output state), control is performed suchthat the output voltage is maintained constant.

The on-time generation circuit 22 has substantially the sameconfiguration as the delay time generation circuit 160 relating toModification 1 of Embodiment 1. Specifically, the on-time generationcircuit 22 detects turn-on of the switching element 2, and then outputsa turn-off signal after elapse of a certain time, namely, an on-time.

According to the switching power supply apparatus relating to thepresent embodiment, the on-time of the switching element 2 is cyclicallymodulated within a time range of a first time to a second time inaccordance with the signal Jitter output from the modulation signalgeneration circuit 13, thereby to modulate the peak value of the draincurrent to modulate the switching frequency.

As described above, the switching power supply apparatus relating to thepresent embodiment exhibits substantially the same effects as those ofthe switching power supply apparatuses relating to Embodiment 1 andModifications 1 to 4 of Embodiment 1.

Alternatively, like in Modification 1 of Embodiment 1, the peak value ofthe drain current may be modulated by providing a certain delay time fora turn-off signal and modulating the delay time. Further alternatively,like in Modification 4 of Embodiment 1, the switching frequency may bediscretely varied by the count-up circuit with use of the modulationsignal generation circuit 13.

Embodiment 3

The following describes a switching power supply apparatus relating toEmbodiment 3 of the present disclosure.

FIG. 15 is a circuit diagram of the switching power supply apparatusrelating to present embodiment.

The switching power supply apparatus relating to the present embodimentbasically has the same circuit configuration as that of the switchingpower supply apparatus relating to Embodiment 1. The switching powersupply apparatus relating to the present embodiment differs from theswitching power supply apparatus relating to Embodiment 1 in terms ofthat a TR terminal is used instead of the FB terminal, an off-timing ofa secondary current is detected by the TR terminal, and the on-duty ofthe secondary current is adjusted in a certain load range such that theload state maintains an output current at a constant value.

A rectifying and smoothing circuit 4 which is configured by a rectifyingdiode 4 a and a smoothing capacitor 4 b is connected with an auxiliarycoil 1 c. The rectifying and smoothing circuit 4 rectifies and smoothesan auxiliary AC voltage, which is induced in the auxiliary coil 1 c dueto a switching operation of a switching element 2, to convert to anauxiliary power source voltage VCC, and supplies the auxiliary powersource voltage VCC to a VCC terminal of a control circuit 3. In otherwords, the rectifying and smoothing circuit 4 is used as an auxiliarypower unit of the control circuit 3.

Also, two resistors 25 and 26 are connected with the auxiliary coil 1 c,and a TR terminal is connected with a connection point between theresistors 25 and 26. Accordingly, a TR terminal voltage VTR whichresults from dividing the auxiliary AC voltage is applied to the TRterminal. As described below, the control circuit 3 detects anoff-timing of a secondary current based on the TR terminal voltage VTR.The off-timing indicates a timing when the secondary current, whichstarts flowing through the secondary coil 1 b of the transformer 1 uponturn-off the switching element 2, stops flowing.

The following describes a control circuit 3.

The control circuit 3 controls the switching operation of the switchingelement 2 to make the output current of the switching power supplyapparatus constant in a certain load range, such that an on-duty ratioof a first time (on-time of the secondary current) to a third time whichis a sum of the first time and a second time (off-time of the secondarycurrent) is constant, that is, such that an on-duty of the secondarycurrent is constant. Also, the control circuit 3 cyclically modulatesthe switching frequency of the switching element 2 to reduce noise.

A secondary current on-time detection circuit 23 is connected with theTR terminal. The secondary current on-time detection circuit 23 detectsan on-time of the secondary current based on the TR terminal voltage VTRand a signal generated in the control circuit 3, thereby to generate adetection signal D2_ON indicating the on-time of the secondary current.The detection signal D2_ON is a logic signal whose signal level is highin the on-time of the secondary current.

Specifically, the secondary current on-time detection circuit 23 detectsa timing at which the secondary current starts flowing (on-timing of thesecondary current) based on the signal generated in the control circuit3, and also detects an off-timing of the secondary current based on theTR terminal voltage VTR.

In other words, according to the switching power supply apparatus of theflyback type, in the on-time of the switching element 2, a current flowsthrough the primary coil 1 a of the transformer 1 and energy is storedin the transformer 1. Also, in the off-time of the switching element 2,the energy stored in the transformer 1 is released and a current(secondary current) flows through the secondary coil 1 b of thetransformer 1. Then, when the value of the secondary current reacheszero, resonance phenomenon occurs due to an inductance of thetransformer 1 and a parasitic capacitance of the switching element 2.This resonance phenomenon occurs in each of the coils of the transformer1. According to the present embodiment, for this reason, the TR terminalis connected with the auxiliary coil 1 c, and the secondary currenton-time detection circuit 23 detects, as an off-timing of the secondarycurrent, a falling timing which firstly appears in a waveform of anauxiliary AC voltage after turn-off of the switching element 2 (timingof polarity inversion of the voltage). Note that, in the case where theTR terminal is connected with the primary coil 1 a instead of theauxiliary coil 1 c, the secondary current on-time detection circuit 23may detect a timing of polarity inversion of the voltage which firstlyappears in the primary coil 1 a after turn-off of the switching element2.

The secondary current starts flowing when the switching element 2 turnsoff. Accordingly, the secondary current on-time detection circuit 23detects, as the on-timing of the secondary current, the turn-off timingof the switching element 2. Here, the secondary current on-timedetection circuit 23 detects falling of a drive signal generated by agate driver 19.

A secondary current on-duty control circuit 24 oscillates a clock signalSet for turning off the switching element 2 based on the detectionsignal D2_ON indicating the on-time of the secondary current, so as tomaintain the on-duty of the secondary current at a predetermined value(a constant value). Each time the clock signal Set rises, the RSflip-flop circuit 17 switches to a set state and the switching element 2turns off. The switching frequency of the switching element 2 isdetermined based on this clock signal Set. As the current flowingthrough the load 7 increases and the on-time of the secondary currentincreases, the frequency of the clock signal Set decreases.

Here, as a conventional frequency modulation method, the on-duty ismodulated by a circuit which includes a low-frequency oscillator thatoscillates at a frequency sufficiently lower than the switchingfrequency of the switching element 2. The circuit generates a currentsignal Jitter (modulation signal) having the lower frequency, andoperates the current signal Jitter so as to maintain a constant on-dutyof a secondary current of a secondary current on-duty control circuit.According to this method, however, there is a case where a clock signalSet for determining a turn-on timing and a modulation signal cancel eachother and as a result modulation effects are offset, like under PFMcontrol. For this reason, the signal Vis, which is output from the draincurrent detection circuit, or the reference voltage, which is input atthe minus side of the comparator 14 included in the drain currentlimiting circuit, is cyclically modulated in the same manner as that inEmbodiment 1.

Alternatively, a delay time may be given for a turn-off signal outputfrom the drain current limiting circuit thereby to cyclically modulatethe delay time.

According to the modulation method described above, the switchingfrequency of the switching element 2 is cyclically modulated whilemaintaining the on-duty at a predetermined value, by applying modulationcomponents to the peak value of the drain current which is maintained ata predetermined value (constant value). Therefore, the effects areexhibited which are substantially the same as those in Embodiment 1.

Also, modulation may be performed with use of the turn-off controlcircuit 150 such as described in Modifications 1 and 2 of Embodiment 1.Also, the same effects are exhibited with use of the configuration ofon-time modulation such as described in Embodiment 2. Moreover, theswitching frequency may be discretely modulated with use of themodulation signal generation circuit 13 such as described inModification 4 of Embodiment 1.

Although the switching power supply apparatus relating to the presentdisclosure has been described based on the embodiments, the presentdisclosure is not limited by the embodiments. Without departing from thespirit of the present disclosure, the present disclosure includesembodiments to which various modifications made by a person skilled inthe art are applied, embodiments configured by combination of componentsof different embodiments, and so on.

In the above embodiments, the semiconductor device is constituted byintegrating the switching element 2 and the control circuit 3 on thesame substrate. However, it is not particularly necessary to integratethe switching element 2 and the control circuit 3 on the same substrate.

In the above embodiments, a current is extracted from the FB terminal asa feedback signal output from the output voltage detection circuit 5.Alternatively, a current may be injected to the FB terminal. Also, thefeedback signal may be detected from the voltage of the VCC terminalinstead of from the output voltage of the secondary side.

In the above embodiments, the low-frequency oscillator 50 includes theresistor for determining the amplitude voltage of the triangular wave.Alternatively, a terminal may be provided in the control circuit 3 so asto externally adjust the resistor. Furthermore, regarding the capacitorfor determining the cycle, a terminal may be provided in the controlcircuit 3 so as to externally connect the capacitor.

Also, the transformer 1 may include the primary coil 1 a, the secondarycoil 1 b, and the auxiliary coil 1 c where the primary coil 1 a and thesecondary coil 1 b are equal in polarity to each other. In this case,the switching power supply apparatus is of a forward type.

INDUSTRIAL APPLICABILITY

According to the switching power supply apparatus and the semiconductordevice relating to the present disclosure as described above, it ispossible to stably and effectively reduce sounds generated by componentssuch as a transformer and a ceramic capacitor and an average value ofterminal noise in the switching power supply apparatus under PFMcontrol, without deteriorating power efficiency compared withconventional arts. This allows the switching power supply apparatushaving a limited operating frequency range for fear of sound generationand so on to perform frequency operations in a wide operating frequencyrange.

Also, the switching power supply apparatus and the semiconductor devicerelating to the present disclosure are utilizable for switching powersupply apparatuses of AC-DC converters, DC-DC converters, and the like.

REFERENCE SIGNS LIST

-   -   1 transformer    -   1 a primary coil    -   1 b secondary coil    -   1 c bias auxiliary coil    -   2 switching element    -   3 control circuit (semiconductor device)    -   4 rectifying and smoothing circuit    -   4 a and 6 a rectifying diode    -   4 b, 6 b, 67, 106, and 155 smoothing capacitor    -   5 output voltage detection circuit    -   6 output voltage generation unit    -   7 load    -   8 regulator    -   9 internal circuit voltage source    -   10 start and stop circuit    -   11 feedback signal control circuit    -   12 PFM control circuit    -   13 modulation signal generation circuit    -   14, 69, 107 and 108 comparator    -   15 AND circuit    -   16 on-time blanking pulse generation circuit    -   17 and 112 RS flip-flop circuit    -   18 NAND circuit    -   19 gate driver    -   20 drain current detection circuit    -   21 drain current limiting circuit    -   22 on-time generation unit    -   23 secondary current on-time detection circuit    -   24 secondary current on-duty control circuit    -   25, 26, 52, 68, 80, 91, 109, 110, and 117 resistor    -   27 auxiliary coil voltage dividing circuit    -   50 low-frequency oscillator    -   51 and 81 NPN bipolar transistor    -   53, 54, 62, 63, 70, 73, 74, 102, 103, 114, 115, 153, 206, and        208 P-MOSFET    -   66, 113, 130, 131, 132, 133, 151, 156, 157, 201, and 203        inverter circuit    -   64, 65, 75, 76, 77, 79, 104, 105, 116, 154, 158, and 159        N-MOSFET    -   59, 60, 61, 71, 72, 101, 152, 205, and 207 constant current        source    -   78 and 82 constant voltage source    -   90 operational amplifier    -   100 oscillator    -   118 V-I converter    -   134 NOR circuit    -   150 turn-off control circuit    -   160 delay time generation circuit    -   202 and 204 D flip-flop circuit

1. A switching power supply apparatus comprising: a transformer thatincludes a primary coil and a secondary coil; a switching element thatis connected in series with the primary coil; a control circuitconfigured to control a switching operation of the switching element toperform switching control on a first DC voltage that is input to theswitching element via the primary coil; an output voltage generationcircuit configured to convert an AC voltage induced in the secondarycoil to a second DC voltage, and supply an output current to a load; andan output state detection circuit configured to output a feedback signalin accordance with the second DC voltage of the secondary coil or theoutput current of the secondary coil, wherein: the control circuitincludes: a turn-on signal generation circuit configured to generate aturn-on signal of the switching element, the turn-on signal being cyclicpulse signals, an oscillation frequency of which is adjusted, when thecyclic pulse signals are output, in accordance with the feedback signalsuch that the second DC voltage or the output current is constant; and aturn-off signal generation circuit configured to generate, independentlyfrom the feedback signal, a turn-off signal of the switching element,and the turn-off signal generation circuit includes: a current detectioncircuit configured to detect a current flowing through the switchingelement and output a detection value corresponding to the current; amodulation signal generation circuit configured to generate a modulationsignal that is a periodic signal; and a timing modulation circuitconfigured to modulate a generation timing of the turn-off signal inaccordance with the modulation signal.
 2. A semiconductor device forcontrolling the switching power supply apparatus of claim 1, wherein thecontrol circuit is formed on a semiconductor substrate as an integratedcircuit.
 3. The switching power supply apparatus of claim 1, wherein:the turn-on signal generation circuit includes: a feedback signalcontrol circuit configured to output a frequency setting signal inaccordance with the feedback signal, the frequency setting signal beingfor setting the oscillation frequency; and a PFM control circuitconfigured to generate the cyclic pulse signals, of which oscillationfrequency is adjusted in accordance with the frequency setting signal,the turn-off signal generation circuit further includes: a currentcontrol circuit configured to, when the detection value reaches a firstreference value, generate the turn-off signal, and the timing modulationcircuit cyclically modulates a peak value of the current flowing throughthe switching element within a range of a first current value to asecond current value.
 4. The switching power supply apparatus of claim3, wherein the timing modulation circuit modulates the first referencevalue within a range of a second reference value to a third referencevalue.
 5. The switching power supply apparatus of claim 3, wherein thetiming modulation circuit modulates the detection value within a rangeof a first detection value to a second detection value.
 6. The switchingpower supply apparatus of claim 3, wherein: the current control circuitincludes a turn-off delay circuit configured to delay generation of theturn-off signal by a first delay time after the detection value reachesthe first reference value, and the timing modulation circuit modulatesthe first delay time within a range of a second delay time to a thirddelay time.
 7. The switching power supply apparatus of claim 1, wherein:the turn-on signal generation circuit includes: a feedback signalcontrol circuit configured to output a frequency setting signal inaccordance with the feedback signal, the frequency setting signal beingfor setting the oscillation frequency; and a PFM control circuitconfigured to generate the cyclic pulse signals, of which oscillationfrequency is adjusted in accordance with the frequency setting signal,the turn-off signal generation circuit further includes: an on-timegeneration circuit configured to set an on-time to a first on-time thatis a constant value, the on-time being a time in which the current flowsthrough the switching element; and a current control circuit configuredto, when the on-time reaches the first on-time, generate the turn-offsignal, and the timing modulation circuit modulates the first on-timewithin a range of a second on-time to a third on-time.
 8. The switchingpower supply apparatus of claim 1, wherein: the turn-on signalgeneration circuit includes: a secondary current on-time detectioncircuit configured to detect a first time in which a secondary currentflows through the secondary coil after the switching element turns off;and a secondary current on-duty control circuit configured to generatethe cyclic pulse signals such that a ratio of the first time to a thirdtime is constant, the third time being a sum of the first time and asecond time in which the secondary current does not flow through thesecondary coil, the turn-off signal generation circuit further includesa current control circuit configured to, when the detection valuereaches a first reference value, generate the turn-off signal, and thetiming modulation circuit cyclically modulates a peak value of thecurrent flowing through the switching element within a range of a firstcurrent value to a second current value.
 9. The switching power supplyapparatus of claim 8, wherein the timing modulation circuit modulatesthe first reference value within a range of a second reference value toa third reference value.
 10. The switching power supply apparatus ofclaim 8, wherein the timing modulation circuit modulates the detectionvalue within a range of a first detection value to a second detectionvalue.
 11. The switching power supply apparatus of claim 8, wherein: thecurrent control circuit includes a turn-off delay circuit configured todelay generation of the turn-off signal by a first delay time after thedetection value reaches the first reference value, and the timingmodulation circuit modulates the first delay time within a range of asecond delay time to a third delay time.
 12. The switching power supplyapparatus of claim 1, wherein: the turn-on signal generation circuitincludes: a secondary current on-time detection circuit configured todetect a first time in which a secondary current flows through thesecondary coil after the switching element turns off; and a secondarycurrent on-duty control circuit configured to generate the cyclic pulsesignals such that a ratio of the first time to a third time is constant,the third time being a sum of the first time and a second time in whichthe secondary current does not flow through the secondary coil, theturn-off signal generation circuit further includes: an on-timegeneration circuit configured to set an on-time to a first on-time thatis a constant value, the on-time being a time in which the current flowsthrough the switching element; and a current control circuit configuredto, when the on-time reaches the first on-time, generate the turn-offsignal, and the timing modulation circuit modulates the first on-timewithin a range of a second on-time to a third on-time.
 13. The switchingpower supply apparatus of claim 1, wherein: when a consumption currentof the load is equal to or higher than a predetermined value, theswitching power supply apparatus operates under PWM control orquasi-resonant control, and when the consumption current of the load isless than the predetermined value, the switching power supply apparatusoperates at the oscillation frequency in accordance with the feedbacksignal.
 14. A switching power supply apparatus comprising: a transformerthat includes a primary coil and a secondary coil; a switching elementthat is connected in series with the primary coil; a control circuitconfigured to control a switching operation of the switching element toperform switching control on a first DC voltage that is input to theswitching element via the primary coil; an output voltage generationcircuit configured to convert an AC voltage induced in the secondarycoil to a second DC voltage, and supply an output current to a load; andan output state detection circuit configured to output a feedback signalin accordance with the second DC voltage of the secondary coil or theoutput current of the secondary coil, wherein: the control circuitincludes: a turn-on signal generation circuit configured to generate aturn-on signal of the switching element, the turn-on signal generationcircuit including an oscillator configured to generate, as the turn-onsignal, cyclic pulse signals having an oscillation frequency which isadjusted in accordance with the feedback signal such that the second DCvoltage or the output current is constant; and a turn-off signalgeneration circuit configured to generate, independently from thefeedback signal, a turn-off signal of the switching element, and theturn-off signal generation circuit includes: a current detection circuitconfigured to detect a current flowing through the switching element andoutput a detection value corresponding to the current; a modulationsignal generation circuit configured to generate a modulation signalthat is a periodic signal; and a timing modulation circuit configured tomodulate a generation timing of the turn-off signal in accordance withthe modulation signal.
 15. The switching power supply apparatus of claim14, wherein: the turn-on signal generation circuit further includes afeedback signal control circuit configured to output a frequency settingsignal to the oscillator in accordance with the feedback signal, and theoscillator generates the turn-on signal according to the frequencysetting signal.
 16. The switching power supply apparatus of claim 14,wherein: the turn-off signal generation circuit further includes acurrent control circuit configured to, when the detection value reachesa first reference value, generate the turn-off signal, and the timingmodulation circuit cyclically modulates a peak value of the currentflowing through the switching element within a range of a first currentvalue to a second current value.
 17. The switching power supplyapparatus of claim 16, wherein: the current control circuit includes aturn-off delay circuit configured to delay generation of the turn-offsignal by a first delay time after the detection value reaches the firstreference value, and the timing modulation circuit modulates the firstdelay time within a range of a second delay time to a third delay time.18. The switching power supply apparatus of claim 14, wherein: theturn-off signal generation circuit further includes: an on-timegeneration circuit configured to set an on-time to a first on-time thatis a constant value, the on-time being a time in which the current flowsthrough the switching element; and a current control circuit configuredto, when the on-time reaches the first on-time, generate the turn-offsignal, and the timing modulation circuit modulates the first on-timewithin a range of a second on-time to a third on-time.
 19. A switchingpower supply apparatus comprising: a transformer that includes a primarycoil and a secondary coil; a switching element that is connected inseries with the primary coil; a control circuit configured to control aswitching operation of the switching element to perform switchingcontrol on a first DC voltage that is input to the switching element viathe primary coil; an output voltage generation circuit configured toconvert an AC voltage induced in the secondary coil to a second DCvoltage, and supply an output current to a load; and an output statedetection circuit configured to output a feedback signal in accordancewith the second DC voltage of the secondary coil or the output currentof the secondary coil, wherein: the control circuit includes: a turn-onsignal generation circuit configured to generate a turn-on signal of theswitching element, the turn-on signal being cyclic pulse signals, anoscillation frequency of which is adjusted, when the cyclic pulsesignals are output, in accordance with the feedback signal such that thesecond DC voltage or the output current is constant; and a turn-offsignal generation circuit configured to generate, independently from thefeedback signal, a turn-off signal of the switching element, theturn-off signal generation circuit includes: a current detection circuitconfigured to detect a current flowing through the switching element andoutput a detection value corresponding to the current; a modulationsignal generation circuit configured to generate a modulation signalthat is a periodic signal; and a timing modulation circuit configured tomodulate a generation timing of the turn-off signal in accordance withthe modulation signal, the timing modulation circuit includes acomparator, and a reference value is input to one of input terminals ofthe comparator, the detection value output from the current detectioncircuit is input to another of the input terminals of the comparator,and the detection value is modulated in accordance with the modulationsignal.
 20. A semiconductor device for controlling the switching powersupply apparatus of claim 19, wherein the control circuit is formed on asemiconductor substrate as an integrated circuit.
 21. The switchingpower supply apparatus of claim 19, wherein: the turn-on signalgeneration circuit includes: a feedback signal control circuitconfigured to output a frequency setting signal in accordance with thefeedback signal, the frequency setting signal being for setting theoscillation frequency; and a PFM control circuit configured to generatethe cyclic pulse signals, of which oscillation frequency is adjusted inaccordance with the frequency setting signal, the turn-off signalgeneration circuit further includes a current control circuit configuredto, when the detection value reaches a first reference value, generatethe turn-off signal, and the timing modulation circuit cyclicallymodulates a peak value of the current flowing through the switchingelement within a range of a first current value to a second currentvalue.
 22. The switching power supply apparatus of claim 19, wherein:the turn-on signal generation circuit includes: a secondary currenton-time detection circuit configured to detect a first time in which asecondary current flows through the secondary coil after the switchingelement turns off; and a secondary current on-duty control circuitconfigured to generate the cyclic pulse signals such that a ratio of thefirst time to a third time is constant, the third time being a sum ofthe first time and a second time in which the secondary current does notflow through the secondary coil, the turn-off signal generation circuitfurther includes a current control circuit configured to, when thedetection value reaches a first reference value, generate the turn-offsignal, and the timing modulation circuit cyclically modulates a peakvalue of the current flowing through the switching element within arange of a first current value to a second current value.
 23. Theswitching power supply apparatus of claim 19, wherein: when aconsumption current of the load is equal to or higher than apredetermined value, the switching power supply apparatus operates underPWM control or quasi-resonant control, and when the consumption currentof the load is less than the predetermined value, the switching powersupply apparatus operates at the oscillation frequency in accordancewith the feedback signal.